Infared (IR) Sensor Alarm Circuit using TSOP1738

This guide shows you how to build a basic alarm system that uses invisible light beams to detect intruders.

This type of alarm is called an IR alarm because it uses infrared light.

It uses a special chip called a IC 555 to create a beam of infrared light that goes across a room.

If someone walks through the beam, it disrupts the light and triggers the alarm.

This is a fun project to learn about electronics, but for a real home security system, it is best to buy one from a store.

WARNING: Security systems can be complex and may not be effective on their own.

What is a Infared (IR) Sensor Alarm Circuit:

An Infrared IR Sensor Alarm Circuit is an electronic system designed to detect the presence of infrared radiation and trigger an alarm or alert mechanism in response to specific conditions.

Typically, these circuits use IR sensors, such as infrared motion detectors or IR receivers to sense changes in the infrared radiation level within their detection range.

The circuit is commonly employed for security applications, detecting movement or intruders in a given area and activating an alarm to alert users or security personnel.

Circuit Working:

Parts List:

The foundation of the circuit lies in the IC 555 monostable multivibrator operating in a stable state until triggered.

The formula for the output pulse duration at pin 3 is determined by the resistor R and capacitor C values connected to the IC which is mentioned below.

To activate the IC 555 monostable a BC547 transistor serves as a switch momentarily triggering pin 2 through a capacitor.

This action involves the charging and discharging of the capacitor, creating a precise trigger.

A crucial element in maintaining standby mode the second BC547 transistor keeps the first transistor off.

This sustains the IC 555 monostable in a deactivated state until an interruption occurs.

The continuous 38 kHz IR signal is produced by another IC 555 configured in astable mode.

The square wave output frequency is determined by resistors R1 and R2 and capacitors C1 and C2.

An IR LED, connected to the astable IC 555 output emits the 38 kHz signal.

The TSOP1738 IC designed to detect this frequency ensures the continuous signal keeps the second BC547 transistor activated.

Any interruption in the 38 kHz IR signal causes the second BC547 transistor to switch off triggering the IC 555 monostable.

The output relay connected to pin 3 activates the alarm.

Formulas and Calculations:

1. The formula for pulse duration:

1.1 * R * C

where,

• 1.1: This approximation factor, which takes into consideration the non ideal behavior of the 555 IC and other circuit parts, can change based on the particular circuit design and component tolerances.
• R: This represents the resistance value in ohms Ω.
• C: This represents the capacitance value in farads F.

Restrictions:

Particular Circuit Design: Only circuits built around a 555 IC set up as a monostable multivibrator can use this formula.

IR sensor alarms with various designs wont be covered by it.

Accuracy: The pulse duration may vary somewhat because of component tolerances and circuit changes; the 1.1 factor is only an estimate.

Example, with R = 10k and C = 1uF, and assuming a factor of 1.1, the formula would estimate a pulse duration of:

Pulse Duration = 1.1 * 10,000 Ω * 1 µF ≈ 0.011 seconds (or 11 milliseconds)

2) Formula for Time Constant:

(τ) = R * C

where,

• (τ): The greek letter tau represents the time constant (τ), which is an important variable in resistor capacitor RC circuits, it shows how long it takes for the voltage across the capacitor C, charged by a voltage source through a resistor R, to get to about 63.2% (1 – 1/e) of its ultimate value.
• On the other hand, it denotes the amount of time it takes for the voltage across the capacitor to drop to about 36.8% (1/e) of its starting value during discharging.
• R: This represents the resistance value in ohms Ω.
• C: This represents the capacitance value in farads F.

3) The formula to calculating the frequency of an astable multivibrator circuit commonly used in IR sensor alarms that utilize a 555 timer integrated circuit.

Frequency = 1.44 / ((R1 + 2 * R2) * C1)

where,

• Frequency: The number of cycles per second that the circuits output voltage oscillates between high and low is known as its frequency.
• Hertz (Hz) is used to measure it.
• R1, R2: These indicate the resistance values in ohms, Ω of the two resistors utilized in the 555 IC timing circuit.
• C1: This is the capacitance value in farads, F of a capacitor that is attached to the 555 IC timing circuit.
• 1.44: is a theoretical constant that derives from the IC 555 circuits characteristics.
• Due to component tolerances and variations, the actual frequency in practical applications may vary significantly.

Using the Formula:

Component Values: This formula can be used to determine the circuits theoretical oscillation frequency if the values for R1, R2, and C1 are specified in your circuit.

Fine Tuning Frequency: R1, R2 or C1 can be adjusted in certain IR sensor alerts.

You may determine the resulting frequency and possibly adjust the operation frequency of the alarm by varying these numbers and applying the formula.

Example:

with R1 = 10k, R2 = 10k, and C1 = 1uF:

• Total Resistance: R1 + 2 * R2 = 10kΩ + 2 * 10kΩ = 30kΩ
• Theoretical Frequency: Frequency = 1.44 / ((30kΩ) * (1µF)) ≈ 48 Hz (assuming ideal component values)

RC Network for IC 555 Monostable:

The IC 555 monostable relies on an RC network where R signifies resistors and C signifies capacitors.

The synergy between these components determines the output pulse duration, a critical factor in the functionality of the intruder alarm system.

The specific selection of resistor and capacitor values is paramount in achieving the desired time delay for activating the alarm.

This customization ensures that the alarm responds promptly and effectively to any potential threat.

Output Relay and Alarm Control:

The output of the IC 555 monostable, emanating from pin 3 establishes a connection with a relay.

The relay functions as a switch responding to the high output produced by the IC when triggered.

The relay assumes a central role in controlling the alarm circuit.

Upon activation it facilitates the flow of power through the alarm circuit generating the predetermined alarm sound or signal.

The duration of the alarm is intricately tied to the timing components within the IC 555 monostable circuit.

IC 555 IR Transmitter:

Parts List:

An additional IC 555, configured as an astable multivibrator serves as the IR transmitter for the continuous 38 kHz signal.

The square wave output frequency is dictated by the carefully chosen values of resistors RV1, R2 and capacitors C1

The RV1 preset must be adjusted carefully to generate 38 kHz from IC 555 pin no 3 and the IR emitter LED.

The square wave output from pin 3 of the astable IC 555 is seamlessly connected to an IR LED.

This component plays a pivotal role in emitting the continuous 38 kHz IR signal, a key element in the uninterrupted operation of the intruder alarm system.

How to Build:

Building the IR signal based alarm circuit involves following step for connections.

IC 555 Section:

• Connect pin 2 of the IC1 555 to the collector of the first BC547 transistor through a capacitor C2.
• Connect pin 6 of the IC1 555 to pin 2 through resistors R1 and R2.
• Connect pin 2 of the IC1 555 to the base of the first BC547 transistor.
• Connect pin 7 to pin 6 through a resistor R3.
• Connect pin 7 of the IC1 555 to the collector of the second BC547 transistor.
• Connect pin 6 of the IC1 555 to the collector of the second BC547 transistor.

IR Signal Section:

• Configure the IC 555 in astable mode.
• Connect pins 6 and 2 to pin 7 through resistors R1 and R2, and connect pin 6 to pin 2 through capacitors C1 and C2.
• Connect pin 3 of the astable IC 555 to the anode of the IR LED.
• Connect the cathode of the IR LED to the positive terminal of the power source.

IR Signal Detection Section:

• Connect the TSOP1738 IR Sensor IC to the circuit with its output connected to the base of the second BC547 transistor.
• Connect the collector of the second BC547 transistor to the base of the first BC547 transistor.

Output Section:

• Connect Pin 3 of the first IC 555 to the relay.
• Connect the relay to the alarm circuit.
• Connect the relay to the power source.

Power Supply:

• Connect the positive and negative terminals of the power source to the respective supply of the PCB.

Testing:

• Apply power to the circuit.
• Ensure the continuous 38 kHz IR signal is emitted from the IR LED.
• Test interruption detection by obstructing the IR signal.
• Verify that the alarm is activated when an interruption occurs.

Important Notes:

• Adjust resistor and capacitor values based on the desired time delays and frequency of the IR signal.
• Ensure proper connections and polarity of components.
• Test the circuit in a controlled environment before deploying it for security purposes.

Safety:

• Remember to take necessary precautions while working with electronic components and if you are not experienced, consider seeking assistance from someone with electronics knowledge.

Conclusion

In conclusion, the intricately designed IR signal based alarm circuit employs an IC 555 monostable configuration triggered by a BC547 transistor ensuring precise control over alarm activation.

The continuous 38 kHz IR signal generated through astable IC 555 operation maintains the standby mode.

Interruptions in the IR signal prompt immediate alarm activation creating a robust security system.

References

How to use 2 TSOP1738 (infrared sensor) in one circuit?

Datasheet TSOP1738